Adding NonZero

[Apersoma]

Adds NonZero as a type in PrimativeInteger

[cuviper]

My initial reaction is that NonZero types could go the same way atomic types did (#24), into its own crate. However, the explicit links with unsigned Div<NonZero> etc. do make it seem appropriate to add here. But if we're going to do that, I would like to try to be complete about it as well, with something like trait NonZeroPrimitiveInteger and Signed/Unsigned sub types, and all their possible methods from the standard library as well.

Are you up for that? I could also experiment with that idea myself.

[Apersoma]

I'm up for it, but I won't have time to do it for a while so feel free to do it yourself.

[Apersoma]

@cuviper
What should T be called on NonZero<T>?
I'm thinking Zeroable, but could also see Inner.

[cuviper]

Zeroable has a nice connection to the standard library ZeroablePrimitive -- although that's unstable and could even be renamed. I suppose that can't be helped.

Integer would probably be OK too.

[Apersoma]

@cuviper
ready for review